The present invention relates generally to torque compensated electric motor and more specifically to their use with rotary consistency transmitters.
Consistency transmitters generally involved an element which is moved relative to the material whose consistency it is to measure. The transmitter senses increased drag in the relative motion between the material and the sensor. This increased drag is a function of the consistency of the material and may be interpreted by appropriate circuitry and provided as an output indication. One type of consistency transmitter is a rotary consistency transmitter wherein a sensor is rotated in the material whose consistency is to be determined. Typically the sensor is rotated by an electric motor. With an increase in the consistency of the material to be sensed, the drag on the sensor is increased. This drag will create a back torque or reaction torque in the stator of the motor. By measuring the reaction or back torque, the consistency of the material is determined.
In prior art rotary consistency transmitters, the stator of the drive motor is free to rotate in response to the back torque or the reaction torque. A torque arm is provided extending radially from the stator. The position of the arm is monitored as an indication of reaction torque. The system is brought into an equilibrium by one of two methods (a) a force balance or (b) a zero displacement. In the force balance system, a counter force is provided to stop the rotation of the torque arm. In the zero displacement, the torque arm is rotated back to a zero displacement position and held at that position. The prior art has sensed the position of the torque arm pneumatically or electrically and has provided in the pneumatic system a bellows to provide the appropriate counter torque. In the electrical system, a linear variable differential transformer has been used to monitor the position of the torque arm and an electric force motor has been used to produce the counter torque. In both these systems, the counter torque producing mechanism is not symmetrical about the axis of rotation of the motor and thus results in undesirable radial forces on the structure which supports the motor.